In recent years, optical measurement techniques, such as a CARS (Coherent Anti-Stokes Raman Scattering) microscope and OCT (Optical Coherence Tomography), have been drawing attention and are expected to be applied to the field of biology or medical care, in particular, due to its non-invasiveness to measurement targets. Conventionally, for analyzing a cell, a method of dying and invading the cell using a reagent and observing the cell with a microscope or the like has been commonly conducted. However, if the aforementioned optical measurement techniques are used, it becomes possible to continuously analyze an identical cell or directly use an inspected cell for medical treatment, for example.
CARS is based on a nonlinear optical phenomenon that when two light beams with different wavelengths are allowed to become incident on an object, a CARS beam is obtained that has a wavelength corresponding to the vibration of molecules forming the object, and is described in Patent Literature 1, for example. A plurality of different methods, such as transmissive CARS and reflective CARS, have been proposed regarding the direction of detecting a CARS beam with respect to the incident direction of a pump beam and a Stokes beam. Non Patent Literature 1 describes, as a feature of reflective CARS, that the dependence on the sample size of the intensity of a CARS beam is large due to the discontinuity of a nonlinear constant, and the intensity rapidly decreases with an increase in the size. Non Patent Literature 1 also describes that, due to such feature, reflective CARS is advantageous for measuring a microscale sample in a medium, such as a culture solution, and that the intensity increases at an interface between two different media, and shows the experimental data in FIG. 14 in which a peak of a reflective CARS signal is obtained at an interface between oil and glass.
Meanwhile, OCT is a method of obtaining shape information, which reflects a change in the refractive index, using interference between a reflected beam from an object and a reference beam that has not irradiated the object, and is described in Patent Literature 2, for example. While a CARS microscope can obtain molecular information on a measurement target, OCT can obtain shape information. Thus, the two techniques are in a mutually complementary relationship. Patent Literature 3 discloses a multi-modal measurement device that combines CARS and OCT, and provides “a measurement device and a measurement method capable of simultaneously measuring structural information and molecular information about a subject.”